Plants become green Mr. Clean to combat toxic messes

The next big way to clean up toxic sites may be coaxing plants to become janitors, a Michigan State University scientist says.

Clayton Rugh, an assistant professor of crop and soil sciences, explains that phytoremediation – using plants to remove contaminants from the soil – is evolving.

In the early stages of this technology, plants were used like sponges, soaking up toxic substances so they can be safely discarded. The next step, Rugh says, is plants that act like a green Mr. Clean, with roots that make detergents to break down toxins. "What were looking at now are kind of Mop & Glo plants," Rugh said. "These plants make detergents that secrete into the soil, making the plant a kind of site custodian."

Rugh spoke today at the American Association for the Advancement of Science annual meeting at a session entitled "Phytoremediation: New Solutions to Pollution on Land and in the Sea."

Rugh is collaborating with colleagues at the Institute of Genetics and Cytology at Minsk, Belarus; the University of York in England; and the Sainsbury Laboratory at Norwich Research Park, U.K., to work on strategies to have plants produce biological detergent compounds – called biosurfactants – that target the thorny environmental problems of hydrophobic pollutants.

Hydrophobic pollutants present some of the most difficult remediation challenges. These are chemicals like PCBs, pesticides and dioxins that cling tightly to soil. They plague the environment because they are persistent, dangerous in small concentrations, and yet are hard to remove. They usually require large-scale, expensive dredging or aggressive chemical or thermal treatments. Hydrophobic– which means water insoluble – pollutants have resisted early attempts at phytoremediation because plants cant readily absorb them.

Rugh and colleagues are having success with genetic engineering to create plants that get to the root of the problem – literally. The rhizosphere is the world that surrounds plant roots, encompassing the bacteria, fungi, "the zone around a plant root that is biologically humming and pulsing with many complex levels of biological interaction," Rugh said.

The trick to make a common laboratory plant – like tobacco – into a janitor is genetic engineering. Rugh said genes from bacteria that naturally produce biological detergents are isolated and inserted into the plants. The plants then gain the ability to release detergents that "ultimately strip the toxic compounds off the soil particles and into the rhizosphere, where they meet their demise. If you change the soil chemistry properly, you really can crank up the phytoremediation process."

The plants and soil microbes can then convert the toxins to more benign chemicals. "Were engineering tobacco plants to treat cancer-causing pollutants," Rugh said. "Now theres some beautiful irony."

This method offers a cheaper, less ecologically disruptive alternative to digging up enormous polluted sites.

"There are sites where we have no choice but to consider such alternatives," Rugh said. "There are places impacted by these chemicals where its impossible to dig them all up, dredge them or burn them. Its not economically possible; it simply will never happen and these sites will continue to be problems for wildlife and people."

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